The present invention relates to improvement of a plasma source mass spectrometer used for quantitative analysis of trace elements in materials or biological fields, and more particularly to improvement of ion extraction means for extracting ions from a plasma generated in a high pressure region.
The conventional plasma source mass spectrometry has been discussed in Anal. Chem., 57, 13 (1985) pp. 2674-2679, Analyst, 108 (February 1983) pp. 159-165, Bunseki, 7 (1985) pp. 505-508, etc. The fundamental construction of the conventional plasma source mass spectrometer is shown in FIG. 2a and the details of a portion A in FIG. 2a is shown by FIG. 2b. In FIG. 2b, reference numeral 10 designates a discharge tube, numeral 20 an inlet for plasma gas, numeral 30 an inlet for sample, numeral 40 an RF (or radio frequency) power supply coil, numeral 50 a plasma, numeral 60 an ion extraction electrode, numeral 61 an aperture provided in the ion extraction electrode 60, numeral 70 a skimmer, numeral 71 an aperture provided in the skimmer 70, numeral 80 an ion extraction lens, numeral 90 an ion beam, and numeral 100 a photon stopper (or baffle). Also, (i) represents a high pressure (.about.1 atm) region, (ii) a moderate pressure (.about.1 Torr) region, and (iii) a low pressure (.ltoreq.10.sup.-4 Torr) region.
In the above-mentioned prior art, no sufficient consideration is paid to the improvement on the efficiency of extraction of ions (sample ions) 90 from the plasma 50 generated in the high pressure (atmospheric pressure) region (i). Therefore, a problem exists in respect of the detection limit (or sensitivity). Also, in order to suppress the deterioration of the S/N ratio of signal (S) to noise (N) caused by intrusion of photons from the plasma into an ion detector, the photon stopper 100 is provided on the axis of the ion beam 90, thereby preventing the photons from entering into the mass analyser side. Therefore, there is a problem that the ion lens system has a complicated construction as shown in FIG. 2a.